1. Field of the Invention
The present invention relates to an objective lens driving device and an optical disc apparatus using the objective lens driving device.
2. Description of Related Art
One type of objective lens driving device for use in an optical disc apparatus is disclosed in Japanese Laid-open Utility Model Application No. Hei-2-35330.
FIG. 9 shows the constitution of the objective lens driving device, FIG. 10 is a cross-sectional view of the objective lens driving device which is taken along a line Gxe2x80x94G of FIG. 9, and FIG. 11 is a view of FIG. 9 which is taken along the direction of an arrow H.
An objective lens 7 is provided in a lens holder 3, which is supported and guided by a sliding shaft 6 which is slidably and rotatably disposed substantially in parallel to the optical axis direction of the objective lens 7. A focusing coil 5 is wound around the outer peripheral surface of the lens holder 3, and tracking coils 4 are provided at predetermined intervals in the peripheral direction on the outer peripheral portion of the lens holder 3. A magnetic circuit comprising a magnet 2, an outer yoke 1 and an inner yoke 9 is provided to generate magnetic flux so that the magnetic flux traverses the tracking coils 4 and the focusing coil 5. The tracking coils 4 and the focusing coil 5 are supplied with current from current supply means such as a flexible printed circuit (hereinafter referred to as xe2x80x9cFPCxe2x80x9d).
During a focus control operation, the electromagnetic force acts in the direction of the sliding shaft 6 by causing the current to flow into the focusing coil 5 in the forward or reverse direction to move the objective lens 7 in the optical axis direction in accordance with the plane vibration of the recording surface of the optical disc, whereby the spot of the optical beam 8 can follow the recording surface of the optical disc. Further, during a tracking control operation, the electromagnetic force acts as a rotational couple of forces on the outer periphery of the sliding shaft 6 by causing the current to flow into each of the tracking coils 4 in the forward or reverse direction, whereby the spot of the optical beam 8 can follow the eccentricity of the track of the optical disc.
Next, the construction of a magnetic spring in a conventional objective driving device will be described by exemplifying an objective lens driving device disclosed in the official gazette for KOKOKU No. Hei-7-31814.
FIGS. 12 and 13 show an objective lens driving device having one objective lens which is used in a conventional CD apparatus or CD-ROM apparatus, wherein FIG. 12 is a diagram showing the constitution of the objective lens driving device and FIG. 13 is a cross-sectional view of the objective lens driving device which is taken along a line Kxe2x80x94K of FIG. 12.
In FIGS. 12 and 13, reference numeral 21 represents an objective lens for focusing an optical beam, reference numeral 22 represents a lens holder, reference numeral 28-1, 28-2 represents a focusing coil, reference numeral 29-1, 29-2 represents a tracking coil, reference numeral 25 represents a sliding shaft, reference numeral 23 represents an inner yoke, reference numeral 24 represents an outer yoke, reference numeral 26-1, 26-2 represents a focusing magnet, reference numeral 27-1, 27-2 represents a tracking magnet, and reference numeral 30-1, 30-2 represents a magnetic substance for positioning the objective lens.
The focusing magnet 26-1, 26-2 is magnetized to have bipolarity in the height direction, and the tracking magnet 27-1, 27-2 is magnetized to have bipolarity in the peripheral direction. The magnetic density distribution in the peripheral direction in the neighborhood of the magnetic substance 30-1, 30-2 which confronts the focusing magnet 26-1, 26-2 is maximized at the center of the magnet, and thus the magnetic substance 30-1, 30-2 is magnetically balanced and stable at the position confronting to the center of the focusing magnet. Paying attention to the flow of the magnetic flux in the height direction, the magnet substance 30-1, 30-2 is magnetically balanced and stable in the neighborhood of the boundary of the N and S poles of the focusing magnet 26-1, 26-2 so as to form a magnetic loop of the N pole of the focusing magnet 26-1, 26-2xe2x86x92the magnetic substance 30-1,30-2xe2x86x92S polexe2x86x92N pole. Accordingly, the objective lens 21 can be stably positioned by the magnetic balance in the rotational direction (tracking direction) and the height direction (focusing direction) with respect to the sliding shaft 25.
Recently, there has been adopted a method of increasing the number of rotation of an optical disc so as to increase the data transfer rate from an optical disc apparatus, and it has become more and more necessary to enhance the follow-up property (responsibility) of the objective lens of the objective lens driving device to the plane vibration and the track eccentricity of the recording surface of the optical disc. Particularly, the acceleration of distortion due to the eccentricity of the track is rapidly increased because it is proportional to the square of the number of rotation, and thus the follow-up performance of the objective lens in the tracking direction is required to be enhanced. A method for intensifying the electromagnetic force of the coil in a gap of a magnetic circuit may be considered to enhance the follow-up performance of the objective lens. In order to intensify the electromagnetic force of the coil, the effective portion of the coil in the gap of the magnetic circuit may be lengthened. However, in the objective lens driving device shown in FIGS. 9 to 11, the lowermost portion of the tracking coil 4 in the sliding axis direction is set to be equal to the lowermost portion of the focusing coil 5 in height. Therefore, in order to prevent the optical beam 8 from being intercepted by the coil when the effective portion of the coil is lengthened, it is necessary that the optical beam 8 is disposed so as to pass over the lower side of the focusing coil 5 and the tracking coil 4 in the sliding shaft direction, or incident from the optical axis direction of the objective lens 7. Consequently, the objective lens driving device is bulky, and thus the optical disc apparatus must be designed in a large size. Further, the optical disc apparatus is also required to be compact in addition to the requirement of the enhancement of the follow-up performance of the objective lens of the objective lens driving device to the plane vibration and the track eccentricity of the recording surface of the optical disc. Therefore, the objective lens driving device is required to be thin and compact. According to the first objective lens shown in FIGS. 9 to 11, if the follow-up performance of the objective lens is enhanced, the objective lens driving apparatus would be bulky (thick). That is, it is impossible to satisfy both the requirement for the enhancement of the follow-up performance of the objective lens and the requirement for the thin (low-profile) and compact design of the objective lens driving device.
According to the present invention, an objective lens driving device comprises a movable unit having an objective lens for focusing an optical beam onto an optical disc, a focusing coil for driving the objective lens in an optical axis direction thereof, a tracking coil for driving the objective lens in a radial direction of the optical disc and a lens holder for holding the objective lens, the focusing coil and the tracking coil, a sliding shaft which is provided substantially in parallel to the optical axis direction of the objective lens and adapted to guide the movable unit, a magnetic circuit for generating magnetic flux which is applied to the focusing coil and the tracking coil, and a mirror for converting the optical beam in the optical axis direction of the objective lens, wherein the tracking coil is disposed outside the objective lens in the radial direction with respect to the sliding shaft, and the whole or part of the optical beam incident to the mirror passes through a space surrounded by the tracking coil and the focusing coil which are adjacent to each other, thereby achieving both the enhancement of the follow-up performance to the optical disc and the thin and compact design of the objective lens driving device.
Recently, the standards of optical discs which have high recording density and are different in substrate thickness have been announced, and they have just started to require optical disc apparatuses which can reproduce not only CDs and CD-ROMs, but also these optical discs having high recording density. Therefore, in order to match both the two types of optical discs which have different recording density or substrate thickness, there may be considered such a type of objective lens driving device that two kinds of objective lens corresponding to the respective types of optical discs are provided and the objective lens being used is switched in accordance with the type of optical disc. In this case, if the magnetic circuit and the positioning mechanism of the objective lens driving device shown in FIGS. 12 and 13 is used, the coil, the magnet and the magnetic substance must be disposed on the same circumference to form a magnetic circuit, and thus the objective lens driving device must be designed in a large shape. Further, it is difficult to use the magnetic circuit commonly, and the number of parts can be expected to increase because these parts must be used exclusively.
The present invention has been implemented in order to solve the above problems, and has an object to provide an objective lens driving device having an objective lens switching mechanism in which the positioning of the objective lens in a focusing direction and a tracking direction can be performed with high reproducibility and high precision even after the objective lens is switched to another, and which can also reduce the number of parts to lower the cost.
Therefore, the objective lens driving device according to the present invention is adapted to the two types of optical discs which are different in recording density, and it includes two types of objective lens which are respectively adapted to focus an optical beam onto an optical disc and are matched with the two types of optical discs, a lens holder for holding a focusing coil and tracking coils for driving the objective lens in the focusing and tracking directions and magnetic substances, and a sliding shaft for sliding and rotating the lens holder in the focusing direction and the tracking direction. Further, the objective lens driving device is provided with a magnet and a yoke which are fixed to confront the coils and keep a magnetic gap to thereby apply magnetic flux to the coils, and the magnetic substances are radially disposed out of the magnetic gap on a plane perpendicular to the plane defining the magnetic gap, whereby a magnetic spring is constituted by leakage flux of the magnetic circuit.
In general, the objective lens driving device is provided with current supply means such as FPC or the like for supplying current to the movable portion comprising the lens holder, the coils, etc. In the objective lens driving device having the objective lens switching mechanism, the current supply means is greatly bent in an interlocking manner with the objective lens switching operation. The restoring force of the current supply means from the bent state to the initial state acts on the movable portion in proportion to the switching amount of the objective lens. Therefore, as the objective lens switching angle is set to a larger value in order to further miniaturize the objective lens driving device, the effect of the restoring force by the current supply means becomes less negligible as comparison with the restoring force of the magnetic spring to the neutral position, and the precision of the positioning of the objective lens is lowered.
The present invention has been implemented to.solve the above problem, and has an object to suppress the lowering of the positioning precision of the objective lens using the restoring force of the current supply means in the objective lens driving device which is provided with plural objective lens and an objective lens switching mechanism.
In order to solve the above problem, according to the present invention, an objective lens driving device includes plural objective lenses for focusing an optical beam, a lens holder for holding the objective lenses, a focusing coil which is provided in the lens holder to drive the objective lenses in a focusing direction, tracking coils which are provided in the lens holder to drive the objective lenses in a tracking direction, a sliding shaft for guiding the lens holder slidably and rotatably, and a yoke and plural magnets for forming a magnetic circuit having a magnetic gap in which the focusing coil and the tracking coils are inserted, wherein magnetic substances are disposed or formed on the lens holder in a radial direction with the sliding shaft located at the rotational center, and the angles at which the respective objective lenses are located around the shaft center of the sliding shaft (i.e., the location angles of the objective lens with the shaft center of the sliding shaft) is set to be varied with respect to the location angles of the magnetic substances around the shaft center of the sliding shaft.